X-ray image converters utilizing lanthanum and gadolinium oxyhalide luminous materials activated with thulium

ABSTRACT

Oxyhalides of lanthanum and gadolinium activated with thulium are found to be superior in their conversion efficiency of X-rays to visible light when compared with conventional X-ray phosphors. These phosphors are used as X-ray image converters generally, and can be used in such devices as X-ray image-intensifier tubes, in fluoroscopic screens and in radiographic intensifier screens.

United States Patent [191 Rabatin Mar. 5, 1974 54] X-RAY IMAGECONVERTERS UTILIZING 3,650,976 /1912 Luckey 250/80 x LQNTHANUM ifi sfi ff gr FOREIGN PATENTS OR APPLICATIONS 0 YHALIDE LU U T 183,308 6/1966U.S.S.R 250/80 [75] Inventor: Jacob G. Rabatin, Chardon, Ohio [73]Assignee: General Electric Company,

V Schenectady, N.Y.

[22] Filed: Aug. 16, 1972 [21] Appl. No.2 281,217

[52] US. Cl 250/460, 250/458, 250/483 [51] int. Cl. H01j l/62 [58] Fieldof Search 250/80, 458, 460, 483v [56] References Cited UNITED STATESPATENTS 3,499,150 3/1970 Tajima et al.., 250/80 ACTIVATED WITH THULIUMPrimary ExaminerArchie R. Borchelt Attorney, Agent, or FirmJohn F.McDevitt; Henry P. Truesdell; Frank L. Neuhauser [5 7 ABSTRACTOxyhalides of lanthanum and gadolinium activated with thulium are foundto be superior in their conversion efficiency of X-rays to visible lightwhen compared with conventional X-ray phosphors. These phos- 5 Claims,No Drawings X-RAY IMAGE CONVERTERS UTILIZING LANTHANUM AND GADOLINIUMOXYI-IALIDE LUMINOUS MATERIALS ACTIVATED WITH TI-IULIUM BACKGROUND OFTHE INVENTION The present invention related to rare earth oxyhalidephosphors activated with thulium for the conversion of X-rays to visiblelight and to related X-ray image converter devices utilizing suchluminescent materials.

In recently issued US. Pat. No. 3,617,743, there is described andclaimed lanthanum and gadolinium oxyhalide luminescent materialsactivated with terbium as a more efficient luminescent material forconverting X-radiation to visible light. Various related image converterdevices utilizing said luminescent materials are also described andclaimed in this patent including X-ray image intensifier tubes,fluoroscopic screens, and radiographic intensifier screens. U.S. Pat.Nos. 3,591,516 and 3,607,770 disclose various processes which can beused to prepare the luminescent material as well as still otherluminescent materialshaving related chemical compositions.

It is also known to prepare a lanthanum oxychloride phosphor activatedwith thulium which emits a blue luminescence when excited by cathoderays. More particularly, USSR Patent 183,308 discloses and claims suchluminescent material along with a method of preparing the phosphor bydirect sintering of the starting materials at temperatures in the range900l,000 C. A starting mixture of La O with NH CL and thulium metal isfirst dried at 100l20 C before the sintering reaction. Experience withthis method has shown that the phosphor product thereby obtainedaverages 1 micron or less in particle size and exhibits approximately1.4 times more lumen brightness than commercial phosphors now in use.

For X-ray image converter applications, especially when used in medicalradiography, it becomes desirable to improve both speed and brightnessof response for conversion of the X-rays to visible light. Calciumtungstate phosphors have been the conventional luminescent material usedin such applications for a number of years. For example, thisconventional fluorescent material has been employed in X-rayintensifying screens for use with photographic film. Typically, suchfilms are placed between two (2) intensifying screens in speciallydesigned cassettes with said phosphor being capable of absorbing theX-rays in the region of interest for conversion to blue-near ultravioletradiation where the photographic film is most sensitive. A faster screenis desirable to reduce the amount of X-ray exposure to the patient, andwith such faster response, further minimizing the occurrence of ablurred image from physical movement by the patient. A more efficientconversion response by the phosphor which produces a visible imagehaving higher brightness at a given level of X-ray exposure is alsodesirable in medical radiography since it enhances the ability tovisually detect an object in the image.

Still, a third important factor in the use of a solid crystallinephosphor for X-ray image converter applications is the size anduniformity of the individual phosphor particle. More particularly,optical scattering effects, which produce a blurred image are caused ifthe individual phosphor particles are below a certain size or if thephosphor particles are irregular in shape. The

most suitable phosphor crystal size range for a 4 mil thick intensifierscreen is not less than about 2 microns in particle size and not morethan about 12 microns in particle size. The optical scatteringcoefficient for a 1 micron size phosphor particle is approximately 4times that for a particle size rangebetween 3 and 4 microns and blurredimages have been experienced with phosphors having a significant numberof particles below 1.5 micron diameter. Much the same undesirable resultcan be experienced if the phosphor particles are sufficiently irregularin shape such that a uniform high packing density cannot be obtained. Inthis latter regard, the phosphor product prepared as disclosed in theaforementioned Russian patent is composed of open structure aggregatessimilar to grapelike clusters which do not pack uniformly and exhibitpacking densities less than 50 percent of the theoretical density.Intensifier screens prepared from such phosphor material yields amottled and grainy image pattern on photographic film which isunsuitable for X-ray image con verter applications.

An important object of the invention, therefore, is to provide animproved phosphor with high sensitivity to X-rays and efficientconversion to near ultraviolet-blue emission. A further object of theinvention is to provide a phosphor having the proper particle shape andsize for optimum packing when employed as an X-ray image converter.

SUMMARY OF THE INVENTION The present invention comprises X-ray imageconverter devices utilizing well-fonned crystals of a materialessentially, according to the formula:

Ln OX: Tm

wherein,

Ln is one or more of La and Gd X is one or more of Cl and Br Tm ispresent as an activator from about 0.05 to 1 mole percent A The abovedescribed phosphors are in the form of a particulate mass having anaverage particle size of approximately 2 microns diameter or greater andhave been found to exhibit approximately 3.3 times the speed of aconventional calcium tungstate phosphor in the 40-100 KeV region of theX-ray spectrum.

Embodiments of the invention include X-ray image converters such asradiographic intensifier screens and fluoroscopic screens, comprising athulium activated phosphor of the invention; X-ray image intensifiertubes comprising an X-ray to visible light converter screen made with athulium-activated phosphor of the invention, a spectrally matchedphoto-emissive surface capable of converting the light image into anelectron image, and a suitable electron-optic system capable of focusingand minifying said electron image onto a second high-resolution phosphorscreen also known as the exit screen, said phosphors of the inventionbased on lanthenum oxychloride or oxybromide or gadolinium oxychlorideor oxybromide further containing between 0.002 and 0.003 moles ofthulium per mole of the phosphor. A more detailed description of thestructural configuration for the various X-ray image converters can befound in the aforementioned US. Pat. No. 3,617,743 patent thus need notbe repeated in connection with the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The applicant has found thatcertain oxyhalides of lanthanum and gadolinium activated with thulium aswell formed and approximately stoichiometric crystals provide efficientconversion of X-ray to visible light.

When such luminescent materials are prepared under conditions which leadto significant deviation from a stoichrometric composition there can bedecreased light production efficiency from poorly crystallized phosphorparticles. Deleterious performance may also be caused by the formationof heterophase crystallites which can be detected by routine X-raydefraction and analysis. As an example, the method of preparing thuliumactivated lanthanum oxychloride phosphor as described in theaforementioned Russian patent utilizes Nl-I Cl as a starting materialwhich at the reactant temperatures produces free ammonia which acts as amoderately strong reducing agent to produce some thulium in the divalentstate and could account for the reduced efficiency. Additionally, theX-ray diffraction studies performed upon said material indicating thatwhile LaOCl is produced as the major phosphor phase by such method,there is present certain unidentified X-ray lines which indicate that aminor second phase in the phosphor is also present.

To prepare the improved stoichiometric phosphors of the presentinvention having both the desirable shape and size distribution foroptimum light conversion in X-ray image converter devices, the two (2)general preferred methods described in the aforementioned patents areemployed. By adapting said general methods with different amounts of athulium precursor material in substitution for a terbium activator therecan be obtained suitable luminescent material for X-ray image converterapplications. For such usage, the luminescent screen can be prepared bydispersing one of the phosphors of the invention in a suitable resingrams of La O are dissolved in 3.02 liters of concentrated HNO Thesolution is diluted to 18 liters with water and subsequently mixed with50 liters of oxalic acid solution containing 5 kilograms of oxalic acid.

5 After filtration, the mixed lanthanum and thulium oxalate precipitateis fired in air for approximately 2 hours which produces a suitablerare-earth oxide mixture to prepare the phosphor. The fired product isthereafter blended with 1,440 grams of NH Br and the blended l0 mixtureis then fired for an additional 2 hours at 400 C. in a covered containerto form the activated phosphor composition. After completing the initialphosphor preparation, a blended mixture is prepared with 531 grams ofKBr and the mixture isthen fired for 2 'hours at approximately 850 C ina covered container under ambient air conditions to obtain arecrystallized final LaoBr: 0.002 Tm product comprising well-formedcrystals having an average particle size of 2 microns or greater. Therecrystallized phosphor is then washed 20 free of KBr, dried and siftedthrough 325 mesh screen size to produce the final X-ray material.

Performance measurements were made upon the above prepared phosphor forcomparison with other prior art phosphors when suspended in a suitableorganic binder system and deposited on a sub-strate to.

' TABLE 1" Absolute Screen x-ray abs. at

Density 80 KV peak Relative Resolution PHOSPHOR qm/cm' l0"bodyfiltration Speed lines per mm CaWo 6.08 .00ll mr 1.0 14 LaOBR.002 Tm6.30 .0023 mr 3.3 14

It can be noted from the above table that for the same screen thickness,the phosphor of the present invention 0 has over twice the X-rayabsorption at 80 KV peak and three times the speed of response ascalcium tungstate. Additionally, there is shown in Table II below theresults in relative speed of response, resolution and relativebrightness for the present phosphor compared with 55 the LAOCI: 0.002 Tmphosphor prepared according to the aforementionedRussian patent as bothare compared with conventional calcium tungstate.

TABLE II Relative Resolution Relative Sample Brightness lines per mmSpeed (Times) LaOCL: .003 Trn 44 ll L45 LaOBr: .002 Tm I00 14 3.3 CaWO.32 7 14 10 form utilizes recrystallization of the phosphor composi tionin an alkaline metal halide. To prepare the starting rare-earth oxidemixture, 5.6 grams Tm O and 2,330

From Table II, it can be observed that while the prior art phosphorexhibits better speed of response than calcium tungstate there isaccompanying poorer resoluin X-ray image converter applications. TableIII below lists various activator concentrations for the same phosphorhost material along with measurements in speed of response under thesame X-ray exposure conditions previously mentioned when compared withcorresponding calciurn tungstate screen.

TABLE Ill Speed relative to Composition CaWO Phosphor It can be seenfrom Table III above that the optimum thulium activator concentrationsare between 0.002 and 0.003 moles thulium per mole of phosphor which ona mole percent basis calculates to a range of 0.2-0.3 mole percent.Comparable results are obtained with GdOBr:Tm and GdOClzTm phosphorsprepared in. accordance with the present invention.

It will be apparent from the foregoing description that an improvedluminescent material has been provided for general use in X-ray imageconverter applications which exhibits-greater light productionefficiency than prior art phosphors. It should also be appreciated fromthe foregoing description that luminescent materials of the presentinvention can be prepared from different starting materials than hereindisclosed, so that it is intended to limit the present invention,therefore, only by the scope of the following claims.

What I claim as new and desire to secure by letters Patent of the UnitedStates is:

1. An X-ray image converter comprising well-formed crystals of amaterial essentially, according to the formula:

Lnoxmn wherein,

Ln is one or more of La and Gd X is one or more of Cl and Br Tm ispresent as an activator from about 0.05 to 1 mole percent, with saidphosphor crystals having an average particle sizeof 2 microns diameteror greater and exhibitng approximately 3.3. times the speed of aconventional calcium tungstate phosphor in the 40-100 KeV region of theX-ray spectrum.

2. An image converter, according to claim 1 comprising a radiographicintensifier luminescent screen which comprises the phosphor supported ona base member, said phosphor being capable of converting X-radiation toradiation of longer wavelength.

3. A luminescent screen, as in claim 2 adapted for intensifiying theexposure of photographic film to X- radiation.

4. An image converter of claim 2 wherein said phosphor is athulium-activated lanthanum oxybromide.

5. An image converter of claim 2 wherein said phosphor is athulium-activated lanthanum oxychloride.

2. An image converter, according to claim 1 comprising a radiographicintensifier luminescent screen which comprises the phosphor supported ona base member, said phosphor being capable of converting X-radiation toradiation of longer wavelength.
 3. A luminescent screen, as in claim 2adapted for intensifiying the exposure of photographic film toX-radiation.
 4. An image converter of claim 2 wherein said phosphor is athulium-activated lanthanum oxybromide.
 5. An image converter of claim 2wherein said phosphor is a thulium-activated lanthanum oxychloride.